In my legally naive phase of game development I was grabbing everything I wanted, and I grew attached to some of it, and I dread having to replace it all.
Never do that. Also on the Open game art site you can ask the artist by posting under the art before using it. You also have several licensees to choose among.
On last thing. Each time you download some art then you take a screenshot of the page, open up a notepad plug in the url and a copy of the licensee it is published under. Then you have something to show your lawyer IF you ever need to go to court. However my experience with Opengameart is that many people there are very fair and honestly.
For a huge bunch of free texture with a very clear license you also have the option of:
http://www.texturemate.com/
that site is cramped with textures. It is published under the CC BY 3.0 except that the sites owner writes that credit is not required
"The stock textures, texture packs, brush packs, and any other resources available for download on this site are completely free and may be used in commercial or non-commercial applications. Credit to texturemate for use of available textures or brushes is appreciated, but not required."
Also bear in mind that:
"The only exception is that they cannot be redistributed commercially in their unedited form. These textures cannot be re-packaged and resold without significant modifications to their appearance. Brush packs may be used to create unique images in Gimp or Adobe Photoshop, but they cannot be redistributed without being significantly edited."
if you can agree to that then you have access to some very nice texture packs and the site's collection keeps on increasing.
The trick is to keep you back safe by keeping some visual proof and then only use websites with clear licensees that are clearly written.
If you need icons:
http://game-icons.net/
Also under the CC BY 3.0
Here you MUST credit the icon designer and the website.
There you go, free icons, texture and art all under the legal public licensees in all shapes and forms but take screenshots and credit where credit should be done and ask where ask should be done. That way you show the designers and artists the proper respect for their hard work
And if you can afford it then donate some for these great websites(it cost some to host all these resources).